Find best selling product for each month

gistfile1.txt

( SELECT productCode, year(orderDate) as orderYear, month(orderDate) as orderMonth
  FROM orderdetails
  JOIN orders on orders.orderNumber = orderdetails.orderNumber
  GROUP BY productCode, YEAR(orderDate), MONTH(orderDate)
  HAVING productCode = ( SELECT productCode
	FROM orderdetails JOIN orders on orderdetails.orderNumber = orders.orderNumber
	WHERE year(orderDate) = orderYear AND month(orderDate) = orderMonth					
	GROUP BY productCode
	ORDER BY COUNT(*) DESC
    LIMIT 1
  )
)